In recent years, the use of ultra-fine metal particles has been major in the production of electronic parts. For example, electrodes of a laminated ceramic condenser are produced by applying and calcining a paste containing ultra-fine Ni particles with an average particle size within a range from 200 to 400 nm.
A lot of production methods of these kinds of ultra-fine metal particles have ever been proposed, and the production method that uses elemental metal as a raw material is disclosed in Japanese Patent Application, First Publication No. 2002-241812.
In this production method, arc discharge is excited in an atmosphere that contains hydrogen, to thereby form the high-temperature arc. In the formed high-temperature arc, a metal material of a raw material is disposed to be melted and evaporated, and then is cooled to thereby obtain ultra-fine metal particles.
Because this production method uses arc discharge, there is the problem that an energy cost increases.
Also, there is the method in which plasma is formed to melt and evaporate a metal material, to thereby produce ultra-fine metal particles. However, there is the problem that an energy cost increases.
Meanwhile, the method that uses a burner is proposed from the point of view of limiting an energy cost. For example, Japanese Unexamined Patent Application, First Publication No. Hei 2-54705 discloses the production method in which air, a fuel such as propane, and a combustion-assisting gas such as oxygen are provided to a burner to form a reducing flame, and a metal compound solution is blown into the reducing flame, to thereby obtain ultra-fine metal particles.
In this production method, the highest temperature of a reducing flame formed by a burner is within a range of 2,700° C. to 2,800° C. (the theoretical flame temperature), and therefore, the metal compound that can be reduced at the aforementioned temperature or lower is used as a raw material.
This is because it has been previously considered that the aforementioned temperature range is not high enough to melt and evaporate elemental metal and it is virtually impossible to melt and evaporate metal powders.
Herein, the theoretical flame temperature refers to the temperature that is obtained using enthalpy balance and element balance when a fuel and a combustion-assisting gas are combusted at an arbitrary ratio in an adiabatic state. The theoretical flame temperature is also referred to as the adiabatic equilibrium flame temperature.
Accordingly, there is not known the method of producing ultra-fine metal particles by using a burner and an elemental metal as a raw material.
[Patent Document 1]
Japanese Unexamined Patent Application, First Publication No. 2002-24812
[Patent Document 2]
Japanese Unexamined Patent Application, First Publication No. Hei 2-54705